What do you mean by "browser wallpaper"?
Themes, formerly known as personas, allow you to change the background of the top toolbars (navigation, bookmarks, etc.) and the bottom toolbar (the add-on bar).
* https://addons.mozilla.org/firefox/themes/
If you want to change the background of the New Tab page, you can use the following add-on.
* https://addons.mozilla.org/firefox/addon/new-tab-tools/